1. Field of the Invention
This invention relates to paravanes for depth and directional positioning of towed seismic source arrays and particularly to a bi-planar pontoon paravane having a pair of wings for directing the flow of water to provide side forces.
2. Description of the Prior Art
In hydrographic seismic surveys over oceans, the state of the art is to tow seismic sources (air-guns) outboard of the towing vessel. Some type of paravane is necessary to apply a force perdendicular to the direction of the vessel's travel and in a plane parallel with the surface of the ocean. This results in the air-gun source arrays being 50 meters to 100 or more meters wide behind the vessel. Configuring in two parallel lines enables graphing two seismic lines at the same pass by shooting guns alternately in their wide-spread configuration behind the vessel, thus cutting survey time and cost in half, and to allow for more than two strings to be towed behind the vessel. Also, by spacing a plurality of paravanes in a geometric pattern, side noise may be attenuated. To be effective for this use, the paravane must have some flotation to maintain the paravane at the surface or a specific depth below the surface.
Commercial fishermen and shrimp boat operators have used paravanes for years to pull their nets outboard of their boats. Their paravanes take the form of simple boards called doors, or otterboards, which have a chain or rope bridle. These types of paravanes are too small to pull the heavy pressure-vessel type of air guns used in the seismic industry and would not withstand the seismic pulses.
United States Navy mine sweeper vessels have employed more sophisticated paravanes with four "air foils" made of formed sheet metal with the sides joined into the classic "box plane". These paravanes provide enough pull for seismic work, but are unwieldy in requiring three separate components to be used: a 500 pound paravane, a 900 pound steel or fiberglass float and the air-gun array itself, which weighs some 5,000 pounds. With this type of paravane, the air-gun arrays still have to be supported by round buoys that create a large drag force.
Another prior art paravane is a long, slender V shaped pontoon that serves as a paravane by its surface area and supports the array of air-guns as well.
Still another prior art paravane looks like a section of an actual airplane wing which is turned on its side. Lead is put in the bottom "tip" of the wing, and the top cavity serves as abuoyancy chamber to maintain the sideways orientation of the paravane. This type paravane tends to speed up, slow down, and jump out of the water at times due to nearly neutral buoyancy.
Still another prior art device is a cylindrical steel pontoon having a steerable wing on the front which steers the pontoon outboard of the ship. This particular prior art device is very large and in fact too large for use on the typical seismic exploration vessel.
To reduce the length of the paravane and to provide a steady, predictable side force, the biplane structure of applicant has been invented.